The present invention relates to capacitor provision in monolithic integrated circuits, and more particularly, to capacitors in monolithic integrated circuits which must have highly accurate values of capacitance over a desired operating range.
Capacitors have been provided in monolithic integrated circuits, both bipolar and field effect transistor based circuits, for much of the time such circuits have been fabricated. The quality of such capacitors in terms of the consistency of the parameters asscciated therewith at fabrication and in various operating evvironments has steadily improved. Even though capacitors based on pn semiconductor junctions can provide a higher value of capacitance in a given area taken up in a major surface of an integrated circuit, the metal-oxdde-semiconductor (MOS) class of capacitors can be fabricated t have parameters that are far more precisely valued.
Furthermore, ratios of capacitances can be maintained to an even more precise values than can the absolute capacitance value of any of the capacitors involved in the ratio. Thus, circuits that are designed depending on ratios of capacitances of the capacitors involved rather than on the absolute capacitance values of such capccitors can provide analog circuit operation which is very accurate and repeatable. Many analog circuits can be constructed in this fashion including analog-to-digital converters, analog gate and attenuation control circuits, and various kinds of filters.
Capacitors of the MOS capacitor class can be formed in a number of alternative structures but all are essentially "parallel plate" capacitors having a pair of conductive, parallel plates separated by some kind of dielectric material. The lower plate of such a capacitor, i.e. the one closest to the interface between the main body of semiconductor material and the first portion of the dielectric material thereover, may be formed of either doped polycrystalline silicon or metal provided in this dielectric material a short way from such interface. Alternatively, such lower plate may be formed ofaa portion of the semiconductor material body which is electrically isolated by either a pn junction or by dielectric material from other portions of such body.
The upper plate, or the plate located in this covering dielectri material somewhat farther from such interface, will be formed from either metal or doped polycrystalline silicon. The dielectric material separating such plates will typically be silicon dioxide, but possibly this dielectric material will be silicon nitride or a combination of these materials or others.
MOS capacitors have improved markedly in the precision of the values for the parameters thereof which result from fabrication and which occur over ranges of various factors in the operating environments. Nevertheless, such capacitors continue to have some dependence on the operating environment such as varying in capacitance value with temperature and applied voltage. As an example, FIG. 1 shows on a normalized basis the variation in capacitance with applied voltage for a MOS capacitor having each plate formed of polycrystalline silicon highly doped with phosphorous. Each plate is separated from the other through use of about 600 .ANG. of silicon dioxide between the plates.
This capacitor, as can be seen from FIG. 1, exhibits a monotonically decreasing, or substantially monotonically decreasing, capacitance value for increasing applied voltage in the selected direction across the capacitor for applying such voltage. The variation in capacitance is on the order of 100 parts per million change in capacitance for a change of 1 volt applied voltage.
The capacitor havingtthe variation shown in FIG. 1 in capacitance with applied voltage is a relatively good capacitor in controlling this parameter. However, advanced circuit designs for achieving extreme accuracy in analog circuit performance require capacitances which exhibit substantially smaller capacitance variation in the operating enviromment. Furthermore, such capacitors should be able to be fabricated with as few changes as possible from the current processes being used to fabricate them.